The recent report by Rasheed, Gardner and Huebner of a rat sarcoma virus derived in tissue culture by the in vitro co-cultivation of a chemically transformed rat cell and a spontaneously transformed rat cell producing rat helper virus, has led to an investigation of the relationship of this sarcoma virus to the Harvey and Kirsten sarcoma viruses. This project has involved both the genetic and biochemical characterization of this rat sarcoma virus.